Some novice questions on H4 status & 485 submission

I am a Canadian citizen currently working in Canada, and my wife is currently working in US under H1B. She submitted her 485 in 2007 and now PD has been current. I know I have the option to file to join once she is greened. Alternatively I guess I may join her under H4 and submit 485 now. With the second option, I have the following questions:

(1) Once I pass the border, an immigration officer told me that I could apply for H4 at the boarder. Is that true?

(2) Once I become H4, can I leave US and continue my emplyment in Canada? Will my revenue from Canadian side be taxed by IRA?

 

Thanks in Advance